Economists normally calculate TFP as the residual of a regression on output against K, L, and maybe other variables.
However, a well behaved regression is expected residuals to be normal (https://en.wikipedia.org/wiki/Normality_test). This is at odds with a situation where TFP increases over time, as in real economies.
Is then the method of residuals flawed? What are some adjustments made to overcome this problem? Maybe adding a trend?